The present invention relates to a reading lens system used in the optics of an apparatus such as a facsimile or an image scanner. More specifically, the present invention relates to a reading lens system that has an aperture ratio of approximately 1:4.5 to 1:5.6 and which features a wide range of half view angles of from approximately 28.degree. to 30.degree..
Reading lens systems used in facsimiles, image scanners and other apparatus of the type that focuses the image of an original at a reduced scale on an imaging device such as CCD are required to have high resolution at the image-forming magnification used and to admit a large quantity of marginal light while producing a small degree of distortion.
The lens systems that have heretofore been proposed with a view to satisfy these requirements include those of the Gauss type, Tessar type and Ernostar type (see, for example, Japanese Patent Publication Nos. 49884/1985 and 51090/1985, and Unexamined Published Japanese Patent Application No. 90810/1984).
The demand of users for reduction in the size of facsimiles and image scanner constantly increases and a need has recently arisen to develop lens systems of Gauss, Tessar or Ernostar type that feature a sufficient narrow view angle to enable shortening of the distance from the original to the imaging device. Cost reduction is another obvious requirement and it has become necessary to reduce the number of lens components.
The prior art lens systems of Gauss and Tessar type are capable of retaining their high performance up to a half-view angle of approximately 20.degree. and if the range is expanded to half-view angle of about 30.degree., it becomes impossible to compensate for astigmatism and curvature of field. Furthermore, at such a large half view angle, the law of cos.sup.4 .omega. causes a dramatic decrease in the quantity of marginal light even if the aperture efficiency is maintained at 100%.
Prior art versions of wide-angle lens systems for use in facsimiles are described in Japanese Patent Publication No. 32852/1985 and Unexamined Published Japanese patent application No. 95416/1985. However, these lens systems have an aperture ratio of 1:2.8 and are costly because of the great number of lens components employed (6-element).